A melting crucible is a pot-shaped container used to hold metal to be smelted in a furnace. They can be made from silicon carbide, graphite, or metals suitable for high temperatures. Crucibles can be cylindrical, conical, or plate-like. Foundries use them to cast small amounts of metal or specialty alloys. The material from which the crucible is made determines the level of temperature it can withstand. Iron and iron alloys are generally not processed using a crucible furnace. Crucibles also experience thermal shock and selecting the right model is important for a foundry’s needs.
A melting crucible is usually a pot-shaped container placed in the center of a furnace that is used to hold a metal to be smelted. It is commonly made from silicon carbide and graphite, but can also be composed of metals suitable for high-temperature applications, such as tantalum or tungsten. Crucibles can also be molded into a conical or planar, plate-like shape, depending on the application.
The industrial crucible is one of the oldest and simplest designs in the metalworking industry. Foundries typically use them to cast small amounts of metal or specialty alloys as opposed to the larger commercial foundry process. Methods for heating the furnace that contains one of several types of crucibles can be anything from firing natural gas or propane to using petroleum, coke, or induction electricity.
Melting crucible shapes are referred to as a bilge for the cylindrical shape and an A shape for the conical shapes, with A crucibles being less expensive as they are easier to manufacture. The material from which the melting crucible is made also determines the level of temperature it can withstand. A silicon carbide crucible can withstand temperatures up to 2,750° Fahrenheit (1,510° Celsius). This makes them suitable for handling all soft metals such as gold, silver and aluminium, as well as alloys such as brass which is mainly composed of zinc and copper.
Iron and iron alloys are generally not processed using a crucible furnace, as the melting point of iron is very high. A standard silicon carbide crucible structure would break and contaminate the metal sample, as iron melts at 2,800° Fahrenheit (1,538° Celsius). A crucible with an admixture of graphite, such as silicon carbide designs, is also not used for iron smelting as the graphite dissolves and reacts with the iron, altering its contents and slowly wearing down the structure of the crucible. Specialized molybdenum metal crucibles can withstand very high temperatures, however, of 4,712° Fahrenheit (2,600° Celsius) and can be used to melt iron.
Furnace designs using a melting crucible are marked by the way the molten metal is removed from the crucible. A holding furnace has a crucible from which molten metal is drawn, while a tilting furnace has a crucible that is tilted to pour the metal. Lift furnaces are designed so that the entire crucible structure can be removed and the molten metal poured or poured.
Aluminum compositions are also common to crucibles, as they can be inert to a wide range of metal chemistries. They work best, however, with a neutral acid level for molten materials. Magnesite crucibles are best suited to basic melt compounds and zircon-silica crucibles to highly acidic compounds.
Crucibles also experience stress as they are repeatedly heated and cooled. This is known as thermal shock, and some metal compounds for crucible walls handle it better than others. The temperatures applied, the chemical compositions of what is being melted, and the amount of sustained use a melting crucible will experience are all important factors to consider in selecting a model best suited to your foundry’s needs.
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